It is common to provide a white light source by applying a yttrium aluminum garnet (YAG) phosphor layer over a blue light emitting diode (LED) die. The YAG phosphor emits a yellow light (also sometimes characterized as a yellow-green light) when energized by the blue light, and the combination of the blue light leaking through the phosphor layer and the yellow light creates white light. Many other colors can be created by combining phosphor light with LED light, and the invention is not limited to YAG phosphor or the use of LEDs.
FIG. 1 is a top down view of a portion of a substantially planar light emitting structure 10. FIG. 2 is a cross-sectional view of a very small portion of the structure 10 along line 2-2 in FIG. 1.
A layer of microscopic bare light emitting diode (LED) dies 12 is printed on a substrate 14, such as a metal substrate with a reflective surface 16, so that the bottom electrodes (e.g., anodes) of the LED dies 12 make electrical contact to a conductor (e.g., the metal substrate 14). A dielectric layer 18 is then printed to fill in the gaps between the LED dies 12, while exposing top electrodes (e.g., cathodes) of the LED dies 12. A transparent conductor layer 20 is then deposited over the LED dies 12 to electrically contact the top electrodes. Thin metal runners (not shown), connected to a metal bus 22, may be printed over the conductor layer 20 to provide a uniform voltage along the conductor layer 20. The metal bus 22 and the substrate 14 may be connected to a voltage source 24 for turning on the LED dies 12.
A YAG phosphor layer 26 is printed over the LED dies 12. The phosphor layer 26 includes YAG particles infused in a binder, and the phosphor layer 26 leaks a certain percentage of the blue LED die light.
In FIG. 1, the LED dies 12 are within the vertical strips 28, and the phosphor layer 26 is deposited over the entire surface of the structure 10.
If the LED dies 12 are GaN types and emit blue light, and the phosphor layer 26 emits yellow light upon being energized by the blue light, the resulting light will appear white.
When the LED dies 12 are in an off-state, white ambient light 30, such as sunlight, impinges on the phosphor layer 26 and energizes the phosphor particles, since the white light includes the wavelengths (e.g., blue) that energize the phosphor. So the entire surface of the light emitting structure 10 appears yellow (or yellow-green). This may not be an aesthetically pleasing color if the light emitting structure is an overhead light, such as a replacement for a conventional fluorescent troffer, where someone directly sees the front surface of the structure 10.
What is needed is a technique for adjusting the off-state color of a phosphor layer, such as a YAG phosphor layer, so the off-state color is more aesthetically pleasing.